Over the last decade, cancer of the prostate has become the most commonly diagnosed malignancy among men and the second leading cause of male cancer deaths in the western population, following lung cancer (Landis et al., 1998, CA Cancer J. Clin. 48(1):6-29). Of all cancers, the incidence of prostate cancer increases most rapidly with age. As longevity among the western population increases, there continues to be a corresponding rise in the number of prostate cancers with an expected increase of 60% in this decade alone. Mortality has increased at a slower rate, but overall has doubled in the last 50 years. Although the disease is typically diagnosed in men over the age of 65, its impact is still significant in that the average life span of a man who dies from prostate cancer is reduced by 9-10 years. If discovered, early prostate cancer can now be cured with surgery in approximately 90% of cases. However, the disease is slowly fatal once the tumor spreads outside the area of the gland and forms distant metastases. Early detection and accurate staging are therefore of great importance for the accurate choice of therapy and should improve the success rate of treatments and reduce the mortality rate associated with prostate cancer.
Despite many advances in recent years, the precision with which an individual suffering from prostate cancer can be staged is still sub-optimal. The main reason for this is the lack of very specific and sensitive molecular tests for accurate staging and the fact that tumor spread beyond the prostate is generally microscopic rather than macroscopic and are therefore difficult to detect. Digital rectal examination of the prostate has been the cornerstone for the local staging of prostatic cancer for many decades, but it oftentimes underestimates the extent of the disease. Transrectal ultrasound by itself is only of limited value as a means of prostate cancer staging. Computer tomography and magnetic resonance imaging have generally been disappointing in the staging of prostate cancer (Kirby, 1997, Prostate cancer and Prostatic Diseases 1:2-10). Recent promising approaches to prostate cancer staging imply the use of biochemical and molecular technologies, centered around proteins markers or their corresponding nucleic acids which are preferentially expressed in prostate cells (Lange, 1997, In “Principles and Practice of Genitourinary Oncology” ed. Lippincott-Raven Publishers, Ch. 41: 417-425).
Tumor markers are often found in a biological sample of cancer patients at elevated concentrations compared to healthy people. These markers are often proteins or nucleic acids encoding such proteins. Tumor markers can also be non-coding nucleic acid molecules. They sometime have the potential to be useful for staging, monitoring and follow up of tumor patients.
The change of the tumor marker level, as well as its value compared to average healthy people has the potential to be used for monitoring cancer therapy. A persistent rise or a value above a defined cut-off can be indicative of recurrent cancer or of a particular stage of cancer. In some cases, tumor makers can also be used for screening persons suspected of having cancer, such tumor markers being often elevated before the appearance of any clinical evidence of the disease.
The identification of tumor markers or antigens associated with prostate cancer has stimulated considerable interest because of their use in screening, diagnosis, prognosis, clinical management and potential treatment of prostate cancer. Indeed, patients with locally confined disease can often be cured by radical prostatectomy or radiation therapy, but for patients with distantly spread disease no curative treatment is available. This emphasizes the need for new prostate (cancer) specific therapeutic targets. Several genes have been described that are specifically expressed in the prostate, e.g., PSA (Sokoll et al., 1997, Prostate-specific antigen. Its discovery and biochemical characteristics. Urol. Clin. North Am. 24:253-259) prostate-specific membrane antigen (PSM: Fair et al., 1997, Prostate-specific membrane antigen. Prostate 32:140-148), prostate stem cell antigen (Reiter et al., 1998. Prostate stem cell antigen: a cell surface marker overexpressed in prostate cancer. Proc. Natl. Acad. Sci. USA 95:1735-1740), TMPRSS2 (Lin et al., 1999. Prostate-localized and androgen-regulated expression of the membrane-bound serine protease TMPRSS2. Cancer Res. 59:4180-4184), PDEF (Oettgen et al., 2000. PDEF, a novel prostate epithelium-specific ets transcription factor, interacts with the androgen receptor and activates prostate-specific antigen gene expression. J. Biol. Chem. 275:1216-1225), prostate-specific gene-1 (Herness, 2003. A novel human prostate-specific gene-1 (HPG-1): molecular cloning, sequencing, and its potential involvement in prostate carcinogenesis. Cancer Res. 63:329-336), and even some non-coding RNA's (ncRNA's), like PCA3 (Bussemakers et al., 1999. DD3: a new prostate-specific gene, highly overexpressed in prostate cancer [Cancer Res. 59:5975-5979], WO98/045420, WO01/023550, WO2004/070056, WO2005/003387), PCGEM1 (Srikantan et al., 2000. PCGEM1, a prostate-specific gene, is overexpressed in prostate cancer. Proc. Natl. Acad. Sci. USA 97:12216-12221) and the gene cluster P704P, P712P, and P775P (Stolk et al., 2004. P704P, P712P, and P775P: A genomic cluster of prostate-specific genes. Prostate 60:214-226). Only a fraction of these genes have been associated with prostate cancer prognosis, progression and/or metastatic capacity and as having the potential to be valuable therapeutic targets. The most notorious prostate tumor markers used for surveillance, follow up, monitoring and choice of therapy for prostate cancer are PSA (prostate specific antigen) and PSM (prostate specific membrane) antigen.
PSA is a serine protease encoded by the PSA gene located on chromosome 19. This glycoprotein is expressed under androgen control by glandular epithelial cells of the prostate and secreted into seminal plasma to liquefy it. PSA protein is normally confined to the prostate but in the case of prostatic disease such as cancer or BPH (benign prostate hyperplasia), PSA leaks into the blood where it is present in different forms, including one that is and one that is not bound to protein complexes (El-Shirbiny, 1994, Adv. Clin. Chem. 31:99). The measurement of total PSA serum concentrations is one of the most frequently used and FDA approved biochemical tests in the screening and management of prostate cancer patients. Studies to date have suggested that screening with PSA, in conjunction with digital rectal exams and transrectal ultrasound, increases the detection of early prostate cancers often while still localized to the gland itself (Brawer et al., 1992, J. Urol. 147:841). Serum PSA is also useful for monitoring of patients after therapy, especially after surgical prostatectomy. However, total PSA measurements also identify a large number of patients with abnormally elevated levels who are subsequently found to have no prostate cancer. Recently, the concept of measuring the percentage free/total PSA ratio was shown to increase the specificity of prostate cancer screening in men with PSA between 4 and 10 ng/ml (Letran et al., 1998, J. Urol. 160:426).
The PSM gene encodes a transmembrane glycoprotein expressed by epithelial cells of normal prostate, benign prostate hyperplasia and, to a greater extent, malignant prostatic tissue. Low levels of PSM are also detected in some other tissues (Israeli et al., 1994, Cancer Res. 54:1807). PSA and PSM have also been targets for molecular approaches to prostate cancer using RT-PCR (reverse transcription-polymerase chain reaction). RT-PCR analyzes of blood, lymph nodes and bone marrow from prostate cancer patients using PSA and PSM have disclosed the extreme sensitivity of this approach. However, further investigations are required to establish the usefulness of PSM as a marker for prostatic cancer.
A new prostate cancer marker, PCA3, was discovered a few years ago by differential display analysis intended to highlight genes associated with prostate cancer development (PCT application number PCT/CA98/00346, and PCT application number PCT/CA00/01154). PCA3 is located on chromosome 9 and composed of four exons. It encodes at least four different transcripts which are generated by alternative splicing and polyadenylation. By RT-PCR analysis, PCA3 expression was found to be limited to the prostate and absent in all other tissues, including testis, ovary, breast and bladder. Northern blot analysis showed that PCA3 is highly expressed in the vast majority of prostate cancers examined (47 out of 50) whereas no or very low expression is detected in benign prostate hyperplasia or normal prostate cells from the same patients. A search of the protein encoded by the putative ORF of PCA3, has yet to be successful. In addition, based on sequence analysis and in vitro translation experiments no protein product was found for PCA3, therefore reinforcing the contention that PCA3 is a non-coding RNA (ncRNA). Thus, although, it is still possible that a polypeptide is encoded by PCA3 (and quickly degraded, processed, etc.), it strongly appears that PCA3 is a ncRNA.
PCA3 would thus be the first non-coding RNA described in relation to prostate cancer. One thing which has been clearly demonstrated, however, is that PCA3 is the most prostate-cancer-specific gene identified to date. PCA3 is alternatively spliced and poly-adenylated and overexpressed 50-500-fold in 95% of prostate cancer tissues and prostate cancer metastases in comparison to normal prostate tissues (de Kok et al., 2002. PCA3, a very sensitive and specific marker to detect prostate tumors. Cancer Res. 62:2695-2698; Hessels et al., 2003. PCA3-based molecular urine analysis for the diagnosis of prostate cancer. Eur. Urol. 44:8-16). No expression is detected in other normal or cancer tissues.
The PCA3 gene is composed of 4 exons (e1-e4) and 3 introns (i1-i3). While PCA3 appears to be recognized as the best prostate-cancer marker ever identified, this specificity has been contested in the literature. For example, Gandini et al., (Cancer Res. 2003; 63(15):4747) claim that the prostate-specific expression of PCA3 is restricted to that of exon 4 of the PCA3 gene. However, the applicants have shown in a recent patent application that this is not the case (WO05/003387). There is at least 20-fold overexpression of PCA3 in prostatic carcinomas in comparison to normal or BPH tissues. Although PCA3 expression seems to increase with tumor grade and is detected in metastatic lesions, a true correlation between PCA3 expression and tumor grade has never been established.
In cancer research it is now well accepted that aggressiveness of cancer is related to the degree of invasiveness of the cancer cell. Hundreds of papers have shown this. Even more, the molecular mechanisms associated with invasion and metastasis become more and more understood. However, these findings appeared restricted to the detection of cancer cells circulating in the blood. The working hypothesis was that invasive cancer cells would migrate into the blood stream and that thus, the number of cancer cells in the circulation would be proportional to the degree of invasiveness of a cancer. Whereas this concept gained a lot of attention more than five years ago, experimental validation has still not been achieved. Thus the concept of measurement of cancer cells in a body fluid such as blood in particular, is still heavily debated.
With the introduction of highly sensitive amplification technologies such as PCR technology which can enable, in some conditions, as little as the detection of a single tumor cell in a background of predominantly normal cells, it became feasible to improve cancer diagnosis in blood samples. It is assumed that transcripts of epithelial cells do not normally occur in the blood circulation. Therefore, the detection of these transcripts in the serum or plasma might indicate the presence of disseminated prostate cancer cells. In the last 12 years many reports have been written on the RT-PCR-based detection of disseminated prostate cancer cells using PSA mRNA as a target. However, remarkable differences were observed in the sensitivity of the RT-PCR-based assays since these assays were qualitative, not standardized, and difficult to reproduce in various laboratories (Foster et al., 2004, Oncogene, 23, 5871-5879). To enhance the sensitivity of these assays researchers used nested-PCR. Unfortunately, this led to the amplification of illegitimate transcripts (Smith et al., 1995, Prostate-specific antigen messenger RNA is expressed in non-prostate cells: implications for detection of micrometastases [Cancer Res. 55: 2640-2644)]. These detected transcripts were produced and secreted in low amounts by any normal cell in the body like normal blood cells or epithelial cells. As a result, PSA mRNA transcripts were found in the serum of women and healthy controls (Henke et al., 1997, Increased analytical sensitivity of RT-PCR of PSA mRNA decreases diagnostic specificity of detection of prostatic cells in blood [Int. J. Cancer. 70: 52-56]). As such, these RT-PCR-based methods were of limited value. New sensitive, quantitative, and more reproducible assays using exogenous internal standards for the detection of PSA and hK2 mRNA transcripts overcame this problem (Ylikoski et al., 2002, Simultaneous quantification of prostate-specific antigen and human glandular kallikrein 2 mRNA in blood samples from patients with prostate cancer and benign disease [Clin. Chem. 48: 1265-127]). However, another problem arose using organ-specific as opposed to cancer-specific transcripts such as PSA mRNA and hK2 mRNA. Indeed, PSA mRNA transcripts were detected in the serum or plasma of men with and without prostate cancer after prostate biopsies, leading to a false-positive indication for the presence of a disseminated cancer cell (Moreno et al., 1997, Transrectal ultrasound-guided biopsy causes hematogenous dissemination of prostate cells as determined by RT-PCR [Urology 49: 515-520] and Polascik et al., 1999, Influence of sextant prostate needle biopsy or surgery on the detection and harvest of intact circulating prostate cancer cells [J. Urol. 162: 749-752]). Thus, there remains a need to identify truly, highly over-expressed and prostate cancer-specific genes which could be used in a quantitative amplification-based assay.
The first suggestion for the appearance of cancer cells in the duct (and thus in a glandular fluid) was provided by Hessel et al., 2003 (Eur. Urol. 44: 8-16). It still remains to be demonstrated whether the relative increase of the number of cancer cells in an organ will correlate with its invasiveness. There also remains a need to show whether the increase in cancer cells in a glandular fluid would correlate with the increase in invasiveness of cancer cells in that gland (e.g., prostate). There also remains to be determined whether such invasiveness would be reflected in the blood, the urine or another body fluid. Indeed, while the hypothesis that an increase of cancer cells in blood (when originating from glandular fluids) should correlate with the grade of cancer has been proposed a long time ago, the clinical validation of that hypothesis remains to be provided.
In view of the fact that prostate cancer remains a life threatening disease reaching a significant portion of the male population, there remains a need for efficient and rapid diagnosis, prognosis and/or theranosis. The development of molecular tests for the accurate staging enabling, amongst other things, the selection of an appropriate therapy, should improve survival rate. However, despite many advances in recent years, the precision with which an individual suffering from prostate cancer can be staged is still sub-optimal. One of the drawbacks of using PSA or PSM for prostate cancer staging is that these markers are expressed in normal as well as in cancerous cells. In addition, poorly differentiated tumors may escape diagnosis since they tend to produce significantly less PSA protein than less aggressive tumors. This is the case for 10% of all prostate cancers.
There thus remains a need to provide a better test for the staging and prognosis of prostate cancer. There also remains a need to provide a prostate cancer test which is more specific and more reliable for prostate cancer detection, staging and treatment methods.
The present invention seeks to meet these and other needs.
The present description refers to a number of documents, the content of which is herein incorporated by reference, in their entirety.